The invention is directed to a process for the continuous enzymatic conversion of water soluble .alpha.-ketocarboxylic acid into the corresponding .alpha.-hydrocarboxylic acid in the presence of a substrate specific dehydrogenase, ammonium ions and nicotinamide-adenine-dinucleotide (NAD.sup.+ /NADH) of increased molecular weight by bonding to a water soluble polymer in a membrane reactor equipped with an ultrafil-tration membrane with simultaneously regeneration of NADH from NADH.sup.+ by means of formate ion in the presence of a formate dehydrogenase.
Enzymatic conversions in membrane reactors indeed have been investigated for a long time but customarily without forced convection of the reaction mixture over the membrane without continuous regeneration of a coenzyme and with very small substrate concentrations. The use of such conversions on a production scale suffered because previously continuous carrying out of the process was not possible and there could only be obtained small space-time-yields.